The purification of plant fumes or automotive emission gases takes place in a gaseous phase reaction with high space velocity and the catalyst carrier used for this purpose is a heat-resistant solid carrier which is catalytically inert. Commonly, a carrier coated with a fireproof metallic oxide such as alumina and treated with a catalytic component such as a metal of the platinum family has been used.
The method of producing the above-mentioned catalyst has been disclosed in U.S. Pat. Nos. 3,264,228, 3,331,787 and 3,565,830, but such a catalyst has been found controversial for the following reasons.
First, the catalytic reaction takes place at high temperatures and accordingly the catalyst bed becomes as hot as 800.degree..about..degree.C. Moreover, through long, continuous use, the catalyst produced by the conventional method has its coating and its catalytic component heavily sintered, resulting in a drastic decrease in its catalytic activity. Secondly, the alumina hydrate coating becomes cracked or chalked through dehydration during firing. Adhesion to the carrier is insufficient with liability to peeling. The use of such a catalyst for the purification of auto emission gas is impossible because the catalyst is exposed continuously to strong vibration and fast gas flow. Thirdly, in the common practice of coating the carrier with a slurry, on account of its strong thixotropy, the viscosity of the slurry increases with time, and it soon gets caked and hardens. Thus it is extremely difficult to keep the amount of the coating and the quality of the coating constant, which makes it impossible to make continuous use of the same slurry, resulting in poor workability and poor efficiency in the utilization of materials, which represents an economic disadvantage.